1. Field of the Invention
The present invention relates in general to a method of forming a desired recessed portion on a surface of a ferrite single crystal, and more particularly to such a method adapted to chemically etch a surface of the ferrite single crystal, so as to produce a magnetic head core having an inclined surface or surfaces, such as a head core for a rigid magnetic disk drive (RDD), or a composite type magnetic head core wherein only a portion adjacent to the magnetic gap of the head core is formed of a magnetic metal.
2. Discussion of the Prior Art
A Mn-Zn ferrite single crystal has been used as a material for magnetic head cores. For the ferrite single crystal to serve its intended purpose, the crystal should be precisely shaped as required.
For instance, a monolithic type head core 1 as illustrated in FIG. 1 is known as a typical magnetic head core for a rigid magnetic disk drive (RDD). In this head core 1, a sliding surface for sliding contact with a recording medium includes air-bearing surfaces 3 and a signal recording/reproducing (writing/reading) track 4, which surfaces 3 and track 4 are defined by grooves 2 formed in the ferrite single crystal. For preventing the edges of the air-bearing surfaces 3 and track 4 from being damaged due to contact with the surface of the recording medium, the edges are chamfered as indicated at 5, 6, 7, 8, 9 and 10. These chamfered or inclined surfaces 5-10 define the nominal widths of the air-bearing surfaces 3 and track 4.
Commonly, the grooves 2 and the chamfered or inclined surfaces 5-10 of the magnetic head core 1 are formed by grinding with a diamond wheel, or by other machining operations. The grooving and chamfering by grinding require a total of eight grinding passes or paths for each head core, and are the most time-consuming step of the process for forming the desired profile. Further, it is inevitable that the widths of the air-bearing surfaces 3 and track 4 have an error of about .+-.10 microns from the nominal values, due to a positioning error of the grinding wheel, and a variation in the thickness or height of the workpiece. Moreover, the surfaces finished by the diamond wheel inevitably suffer from chipping of one micron or more.
Recently, there has been an increasing requirement for improved accuracy of the widths of the air-bearing surfaces 3 and track 4 of the head core, say, to within .+-.1 micron, keeping pace with an increasing demand for improved recording density per unit area of the recording medium, and improved accuracy of air-bearing surfaces 3 and magnetic characteristics of the RDD head core 1. It is also noted that the chipping or cracking of the air-bearing surfaces 3 and track 4 may lead to chipping or removal of the ferrite material, causing damages of the magnetic head core and the recording medium, during an operation of the head core. Thus, the conventional magnetic head core suffers from the problems of reliability, associated with its floating action and magnetic characteristics.
On the other hand, methods of producing such magnetic head cores by means of photolithography or a chemical etching process have been proposed, as disclosed in laid-open Publication No. 62-234214 of unexamined Japanese Patent Application.
Described more specifically, monolithic RDD magnetic head cores are prepared from a so-called ferrite bar 12 formed of a ferrite single crystal, as illustrated in FIG. 2(a). The ferrite bar 12 has a predetermined magnetic gap 11, and an annular magnetic path formed so as to include the gap. The ferrite bar 12 has a surface 13 which corresponds to a sliding surface of each magnetic head core produced. The sliding surface 13 is mirror-ground, and is subjected to a process for removing residual strain due to the grinding operation. Then, a resist 14 is applied to the processed surface of the ferrite bar 12, so as to form an appropriate etching mask in a pattern corresponding to a recessed portion to be formed on the surface 13. The ferrite bar 12 with the etching mask 14 is subjected to a chemical etching operation, whereby the recessed portion, having a predetermined cross sectional profile partly defined by the air-bearing surfaces 3 and track 4, is formed on the ferrite bar 12.
It is desirable that the recessed portion is formed by etching on the surface of the ferrite single crystal have a high degree of dimensional accuracy. Namely, consistent dimensional accuracy is required of side etching widths W and straightness of straight ridges 15, which define the dimensions of the track 4 and chamfered surfaces 7, 8, for example, as indicated in FIG. 2(b). As indicated above, the dimensional accuracy to within .+-.1 micron is important to assure improved recording density, magnetic characteristics and reliability of the magnetic head core.
However, the ferrite bar 12 of a Mn-Zn ferrite single crystal for a magnetic head core has a glass filler at the magnetic gap 11. Since the ferrite bar 12 with the glass filler cannot be heated to a temperature higher than a melting point of the glass, a strain on the surface of the ferrite single crystal caused in the grinding process cannot be eliminated. As a result, the side etching widths W tend to be excessive with respect to the nominal values, and the straightness of the straight edges 15 tends to be deteriorated. Thus, the chemical etching process suffers from difficulty in obtaining acceptable dimensional accuracy of the recessed profile formed on the ferrite bar 12.
Also, dry etching methods such as sputtering and ion-beam etching rather than chemical etching, are proposed to process the ferrite bar 12 to form thereon a recessed portion having a desired cross sectional shape. Examples of such dry etching methods are disclosed in laid-open Publication Nos. 51-114111 and 54-12819 of unexamined Japanese Patent Applications. However, the dry etching methods suffer from a low etching rate, and are difficult to practice in an industrially justifiable manner.